That’s a tough one & unless someone has been through the same experience with the same mind set I’m not sure anyone other than a professional shrink can give a view, but seeing that he seems unwilling to accept that kind of help, here’s a couple of things that might get him back to enjoying his riding..
If you are willing, maybe suggest that you’d love to go a ride out as pillion for a pub lunch or something, nothing too far but some nice country lanes & a river side venue?
Secondly organise a ride out with some mates or find a local group he could join?
He might not be wanting to commute due to a fear of having the bike stolen again … so maybe a bit of leisure riding will get him back in the mood?
…. I am not a mental health professional so I take. I responsibility if the above is a load of bollox.